Offshore LNG regasification has become an increasingly attractive option in LNG import. Among other advantages, offshore regasification terminals or terminals in a relatively remote location help to reduce various safety and security concerns of local communities nearby a terminal that would otherwise be onshore or in a location near human habitation and/or activity.
Unfortunately, offshore installations are generally significantly more expensive than onshore installations, and numerous additional technical challenges arise from offshore LNG storage, unloading and regasification. Several solutions have recently been proposed to overcome at least some of these difficulties. However, all or almost all of the presently known offshore configurations fail to provide a mechanism by which chemical composition of the LNG can be altered to a desirable composition (e.g., processing of lower quality LNG with heating values higher than the North American pipeline specifications). As pipeline transport of natural gas in North America and other countries must typically meet hydrocarbon dew point and gross heating value requirements of the associated distribution systems, presence of heavier components in LNG is generally not desirable.
In many presently known configurations, heavy hydrocarbons are removed from LNG in a process that includes vaporizing the LNG in a demethanizer using a reboiler, and re-condensing the demethanizer overhead to a liquid that is then pumped and vaporized. For example, McCartney describes in U.S. Pat. No. 6,564,579 such regasification process and configurations. While these configurations and methods typically operate satisfactorily under onshore conditions, offshore installation would be unacceptable under most scenarios as these configurations require relatively substantial space.
In currently known offshore LNG regasification terminals, LNG is typically heated to pipeline specification (e.g., about 50° F. and 1200 psig) in offshore vaporizers using seawater or submerged combustion vaporizers. Commonly, fractionation facilities are not provided due to the space limitation in an offshore environment, and the regasified LNG is then sent via an undersea pipeline to an onshore consumer gas pipeline. Thus, while offshore regasification is realized, change in chemical composition is typically not possible using such configurations. It should be noted that when the LNG is fully vaporized, BTU reduction and/or recovery of non-methane components (e.g., ethane, propane, etc.) is generally not economical as these processes would require significant refrigeration and recompression. Consequently, and at least for these reasons, only high quality LNG with acceptable heating value content and/or desirable chemical composition are imported, while lower quality LNG (e.g., LNG with relatively high BTU) is often rejected.
Thus, while numerous configurations and methods to separate heavier components from LNG or to reduce BTU of LNG are known in the art, all or almost all of them fail to provide economically attractive operation, especially in an offshore environment. Therefore, there is still a need to provide improved configurations and methods for LNG regasification that allows for simple and cost-effective removal of non-methane components to thereby produce LNG with a desirable BTU and/or chemical composition.